1. Field of the Invention
The present invention relates to high-frequency transmission lines that are used, for example, in the microwave band or the millimeter wave band. More specifically, the present invention relates to a high-frequency transmission line having a construction that allows a high-frequency signal in an operating transmission mode to be transmitted while suppressing an unwanted mode.
2. Description of the Related Art
Various transmission lines that are used in the microwave band or the millimeter wave band have been proposed. Their transmission lines require that unwanted modes other than an operating mode to be transmitted be suppressed.
For example, H. Yoshinaga and T. Yoneyama, “Design and fabrication of a nonradiative dielectric waveguide circulator”, IEEE Trans. on Microwave Theory and Tech., vol. 36, No. 11, pp 1526-1529, November (1998) discloses a transmission line including a mode suppressor for suppressing an unwanted mode. As shown in FIG. 19, according to the related art, a nonradiative dielectric line includes a metallic plate 101 disposed in a direction of transmission indicated by an arrow A, i.e., a direction that is perpendicular to an electric field associated with an operating transmission mode, so that an unwanted mode will be suppressed. In FIG. 19, 102 denotes an upper metallic plate, 103 denotes a lower metallic plate, and 104 denotes a dielectric strip. The metallic plate 101 is disposed in the dielectric strip 104.
If the LSM01 mode the operating transmission mode and if the LSE01 mode is an unwanted mode, the distributions of electromagnetic field vectors of these modes are as shown in FIGS. 20A and 20B, respectively. FIG. 20A shows the distribution of electromagnetic field vectors in a plane that is perpendicular to the direction of transmission of the LSM01 mode, in which solid lines indicate electric field vectors and dashed lines indicate magnetic field vectors schematically. Similarly, FIG. 20B shows the distribution of electromagnetic fields associated with the LSE01 mode.
The use of the metallic plate 101 allows the unwanted LSE01 mode to be suppressed while not affecting the operating LSM01 mode.
The use of the metallic plate 101, however, causes transmission of the TEM mode. Accordingly, it has been required to suppress the TEM wave by constructing the line so as to form a λg/4 choke structure against the TEM wave.
The IEICE (The Institute of Electronics, Information and Communication Engineers) Trans C-1, Vol. J73-C-1 No.3, pp 87-94 (March, 1990) discloses an attenuator for a radiative dielectric line, which is shown in FIGS. 21A and 21B. Referring to FIGS. 21A and 21B, a resistive film 113 composed of nickel-chromium, having a surface resistivity of 500 Ω/mm2, is disposed between dielectric strips 111 and 112 to form an attenuator. The dielectric strips 111 and 112 are integrated by bonding. A conductor plate (not shown) is disposed on an upper surface of the dielectric strip 111, and a conductor plate is also disposed on a lower surface of the dielectric strip 112.
The resistive film 113 functions as an attenuator that suppresses transmission of the operating LSM01 mode, and a resistive film 101 is disposed in a direction that is parallel to an electric field associated with the LSM01 mode.
The resistive film shown in FIGS. 21A and 21B, however, functions only as an attenuator as described above, and does not serve to suppress an unwanted mode and to thereby transmit the operating transmission mode efficiently.